Geoscience Reference
In-Depth Information
found in higher rock layers, which is why it was so important for us to
determine how many different rock layers contained eggs. Each egg-
bearing layer represents a different nesting event because each rock
layer represents a different page or moment in the history of the site.
So, if we found sauropod eggs in more than one of the rock layers at
Auca Mahuevo, we could be certain that the sauropods had laid their
eggs more than one time at the site.
During our first trip to Auca Mahuevo in 1997, we had discovered
that the eggs on the flats and the eggs in the embryo quarry both came
from a twenty-four-foot-thick layer of mudstone that began about fifty
feet above the lowest layer of rock exposed at the site. At first, we
weren't sure whether the eggs from the flats and the eggs from the
embryo quarry about a mile away actually came from the same layer.
However, after hiking on the mudstone layer that contained the
embryos all the way back to the flats, we were sure that all the eggs
came from the same mudstone layer, leading us to conclude that there
was one enormous nesting site covering a couple of square miles. That
conclusion still stands, and our subsequent investigations showed the
area to be even larger. The evidence we discovered during later trips to
the site, however, documented that the sauropods had laid their eggs
at Auca Mahuevo several different times.
We got our first indication of this in May 1998 before our second
expedition. Lowell, Luis, Rodolfo, Suzi Zetkus, and Pablo Puerto
had briefly returned to the site because
National Geographic
was
planning to do an article on our discoveries. Brooks Walker, a pho-
tographer working for the magazine, accompanied us throughout
our weeklong stay. Most of our time was spent helping Brooks take
photos around the flats and at the embryo quarry for the article, rather
than conducting scientific work. But one day, late in the week, Brooks
climbed up to the top of a small ridge overlooking the site to get some
panoramic shots. The ridge was across the road to Auca Mahuida from
where we had found the eggs on the flats and the embryo quarry. Con-
sequently, we had not spent much time looking for fossils there yet. As
Brooks was taking pictures, he noticed some eggs weathering out of
the ground near the base of a small peak on top of the ridge. After fin-
ishing his photos, he came back down and told us about his discovery.
Of course, we were thrilled to learn that more eggs were present in
a new area of the site. We suspected at first that the eggs were com-
